The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between a magnetic pinned layer structure and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is oriented generally perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is oriented generally parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos ⊖, where ⊖ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. The ferromagnetic layer next to the spacer layer is typically referred to as the reference layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
The CIP spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a CPP spin valve, the sensor is sandwiched between first and second leads which can also function as the shields. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
Magnetization of the pinned layer is usually fixed by exchange coupling one of the ferromagnetic layers (AP1) with a layer of antiferromagnetic material such as PtMn, IrMn, NiMn, or IrMnX (X═Cr). While an antiferromagnetic (AFM) material does not in and of itself have a magnetization, when exchange coupled with a magnetic material, it can strongly pin the magnetization of the ferromagnetic layer.
The ever increasing demand for increased data rate and data capacity has lead a relentless push to develop magnetoresistive sensors having improved signal amplitude and reduced track width. Sensors that show promise in achieving higher signal amplitude at high recording densities are current perpendicular to plane (CPP) sensors. Such sensors conduct sense current from top to bottom, perpendicular to the planes of the sensor layers. Examples of CPP sensors include CPP GMR sensors. A CPP GMR sensor operates based on the spin dependent scattering of electrons through the sensor, similar to a more traditional current in plane (CIP) GMR sensor except that, as mentioned above, the sense current flows perpendicular to the plane of the layers.
However, a problem experienced by CPP GMR sensors is that they suffer from spin torque noise. As those skilled in the art will appreciate, spin torque noise occurs when electrons pass from one magnetic layer to another magnetic layer through a spacer. The polarization of the electrons and the magnetization of the free layer affect one another. For example, the torque from polarized electrons originating from the reference layer can destabilize the magnetization of the free layer, causing spin torque noise, and vice versa. This will adversely affect the signal to noise ratio of a sensor, making the CPP GMR impractical.
One way to avoid spin torque noise is to construct a CPP GMR sensor as a dual sensor having a free layer disposed between two pinned layer structures. Several factors have, however, made such dual spin CPP GMR sensors impractical. For example, in order to provide sufficient pinned layer stability it has been necessary to construct the pinned layers as antiparallel coupled AP pinned layers as described above. This design concept minimizes magneto-static coupling between the pinned layer structures and the free layer. Unfortunately, a CPP GMR suffers a reduction in GMR performance (dR/R) when an AP coupled pinned layer is used. This is because, the magnetic pinned layer furthest from the spacer layer (AP1) has a negative contribution to the GMR effect. As those skilled in the art will appreciate, this is due to the fact that this layer is pinned in a direction opposite to the magnetic pinned layer closest to the spacer layer (AP2). Since a dual CPP sensor has two such AP coupled pinned layers, this problem is even worse in a dual CPP GMR sensor.
Therefore, there is a need for a CPP GMR design that can mitigate the effects of spin torque noise while also maximizing the GMR effect or performance of such a sensor. Such a design would preferably also minimize the negative contribution to GMR that would be provided by an AP coupled pinned layer.